Aspergillus fumigatus Hypoxia Adaptation Is Critical for the Establishment of Fungal Keratitis

Purpose

The poor visual outcomes associated with fungal keratitis (FK) underscore a need to identify fungal pathways that can serve as novel antifungal targets. In this report, we investigated whether hypoxia develops in the FK cornea and, by extension, if fungal hypoxia adaptation is essential for virulence in this setting.

Methods

C57BL/6J mice were inoculated with Aspergillus fumigatus and Fusarium solani var. petroliphilum via topical overlay or intrastromal injection. At various time points post-inoculation (p.i.), animals were injected with pimonidazole for the detection of tissue hypoxia through immunofluorescence imaging. The A. fumigatus srbA gene was deleted through Cas9-mediated homologous recombination and its virulence was assessed in the topical infection model using slit-lamp microscopy and optical coherence tomography (OCT).

Results

Topical inoculation with A. fumigatus resulted in diffuse pimonidazole staining across the epithelial and endothelial layers within 6 hours. Stromal hypoxia was evident by 48 hours p.i., which corresponded to leukocytic infiltration. Intrastromal inoculation with either A. fumigatus or F. solani similarly led to diffuse staining patterns across all corneal cell layers. The A. fumigatus srbA deletion mutant was unable to grow at oxygen levels below 3% in vitro, and corneas inoculated with the mutant failed to develop signs of corneal opacification, inflammation, or fungal burden.

Conclusions

These results suggest that fungal antigen rapidly drives the development of corneal hypoxia, thus rendering fungal SrbA or related pathways essential for the establishment of infection. Such pathways may therefore serve as targets for novel antifungal intervention.

The Aspergillus fumigatus UPR is variably activated across nutrient and host environments and is critical for the establishment of corneal infection

The Aspergillus fumigatus unfolded protein response (UPR) is a two-component relay consisting of the ER-bound IreA protein, which splices and activates the mRNA of the transcription factor HacA. Spliced hacA accumulates under conditions of acute ER stress in vitro, and UPR null mutants are hypovirulent in a murine model of invasive pulmonary infection. In this report, we demonstrate that a hacA deletion mutant (ΔhacA) is furthermore avirulent in a model of fungal keratitis, a corneal infection, and an important cause of ocular morbidity and unilateral blindness worldwide. Interestingly, we demonstrate that A. fumigatus hacA is spliced in infected lung samples, but not in the cornea, suggesting the amount of ER stress experienced by the fungus varies upon the host niche. To better understand how the UPR contributes to fungal cell biology across a spectrum of ER-stress levels, we employed transcriptomics on the wild-type and ΔhacA strains in glucose minimal media (low stress), glucose minimal media with dithiothreitol (high stress), and gelatin minimal media as a proxy for the nutrient stress encountered in the cornea (mid-level stress). These data altogether reveal a unique HacA-dependent transcriptome under each condition, suggesting that HacA activity is finely-tuned and required for proper fungal adaptation in each environment. Taken together, our results indicate that the fungal UPR could serve as an important antifungal target in the setting of both invasive pulmonary and corneal infections.

A host defense peptide mimetic, brilacidin, potentiates caspofungin antifungal activity against human pathogenic fungi

Fungal infections cause more than 1.5 million deaths a year. Due to emerging antifungal drug resistance, novel strategies are urgently needed to combat life-threatening fungal diseases. Here, we identify the host defense peptide mimetic, brilacidin (BRI) as a synergizer with caspofungin (CAS) against CAS-sensitive and CAS-resistant isolates of Aspergillus fumigatus, Candida albicans, C. auris, and CAS-intrinsically resistant Cryptococcus neoformans. BRI also potentiates azoles against A. fumigatus and several Mucorales fungi. BRI acts in A. fumigatus by affecting cell wall integrity pathway and cell membrane potential. BRI combined with CAS significantly clears A. fumigatus lung infection in an immunosuppressed murine model of invasive pulmonary aspergillosis. BRI alone also decreases A. fumigatus fungal burden and ablates disease development in a murine model of fungal keratitis. Our results indicate that combinations of BRI and antifungal drugs in clinical use are likely to improve the treatment outcome of aspergillosis and other fungal infections.

Phylogenomic Analysis of a 55.1-kb 19-Gene Dataset Resolves a Monophyletic Fusarium that Includes the Fusarium solani Species Complex

Scientific communication is facilitated by a data-driven, scientifically sound taxonomy that considers the end-user's needs and established successful practice. In 2013, the Fusarium community voiced near unanimous support for a concept of Fusarium that represented a clade comprising all agriculturally and clinically important Fusarium species, including the F. solani species complex (FSSC). Subsequently, this concept was challenged in 2015 by one research group who proposed dividing the genus Fusarium into seven genera, including the FSSC described as members of the genus Neocosmospora, with subsequent justification in 2018 based on claims that the 2013 concept of Fusarium is polyphyletic. Here, we test this claim and provide a phylogeny based on exonic nucleotide sequences of 19 orthologous protein-coding genes that strongly support the monophyly of Fusarium including the FSSC. We reassert the practical and scientific argument in support of a genus Fusarium that includes the FSSC and several other basal lineages, consistent with the longstanding use of this name among plant pathologists, medical mycologists, quarantine officials, regulatory agencies, students, and researchers with a stake in its taxonomy. In recognition of this monophyly, 40 species described as genus Neocosmospora were recombined in genus Fusarium, and nine others were renamed Fusarium. Here the global Fusarium community voices strong support for the inclusion of the FSSC in Fusarium, as it remains the best scientific, nomenclatural, and practical taxonomic option available.

A novel 3D culture model of fungal keratitis to explore host-pathogen interactions within the stromal environment

Fungal keratitis (FK) pathology is driven by both fungal growth and inflammation within the corneal stroma. Standard in vitro infection models ̶ involving co-culture of the pathogen and the corneal cells in tissue culture medium ̶ are sufficient to probe host responses to the fungus; however, they lack the physiological structure and nutrient composition of the stroma to accurately study fungal invasiveness and metabolic processes. We therefore sought to develop a culture model of FK that would allow for both host and fungal cell biology to be evaluated in parallel. Towards this end, we employed a previously described system in which primary human cornea fibroblasts (HCFs) are cultured on transwell membranes, whereupon they secrete a three-dimensional (3D) collagen matrix that resembles the human stroma. We demonstrated that two common mold agents of FK, Fusarium petroliphilum and Aspergillus fumigatus, penetrated into these constructs and caused a disruption of the collagen matrix that is characteristic of infection. HCF morphology appeared altered in the presence of fungus and electron microscopy revealed a clear internalization of fungal spores into these cells. Consistent with this apparent phagocyte-like activity of the HCFs, mRNA and protein levels for several pro-inflammatory cytokines/chemokines (including TNFα, IL-1β, IL-6, and IL-8) were significantly upregulated compared to uninfected samples. We similarly found an upregulation of several HCF metalloproteases (MMPs), which are enzymes that breakdown collagen during wound healing and may further activate pro-inflammatory signaling molecules. Finally, several fungal collagenase genes were upregulated during growth in the constructs relative to growth in tissue culture media alone, suggesting a fungal metabolic shift towards protein catabolism. Taken together, our results indicate that this 3D-stromal model provides a physiologically relevant system to study host and fungal cell pathobiology during FK.

A Pro- and Anti-inflammatory Axis Modulates the Macrophage Circadian Clock

The circadian clock broadly governs immune cell function, leading to time-of-day differences in inflammatory responses and subsequently, pathogen clearance. However, the effect of inflammatory signals on circadian machinery is poorly understood. We found that in bone marrow-derived macrophages, some host-derived pro-inflammatory cytokines, e.g., IFN-γ or TNF-α, and pathogen-associated molecular patterns, e.g., LPS or Pam3Csk4, suppress the amplitude in oscillations of circadian negative feedback arm clock components such as PER2, and when examined, specific combinations of these immune-related signals suppressed the amplitude of these oscillations to a greater degree in both bone marrow-derived and peritoneal macrophages. At the transcript level, multiple components of the circadian clock were affected in different ways by pro-inflammatory stimulus, including Per2 and Nr1d1. This suppressive effect on PER2 did not arise from nor correlate with cell death or clock resetting. Suppression of the clock by IFN-γ was dependent on its cognate receptor; however, pharmacological inhibition of the canonical JAK/STAT and MEK pathways did not hinder suppression, suggesting a mechanism involving a non-canonical pathway. In contrast, anti-inflammatory signals such as IL-4 and dexamethasone enhanced the expression of PER2 protein and Per2 mRNA. Our results suggest that the circadian system in macrophages can differentially respond to pro- and anti-inflammatory signals in their microenvironments.

Ocular Glands Become Infected Secondarily to Infectious Keratitis and Play a Role in Corneal Resistance to Infection

Ocular glands play a critical role in eye health through the secretion of factors directly onto the ocular surface. The cornea is a normally transparent tissue necessary for visual acuity located in the anterior segment of the eye. Corneal damage can occur during microbial infection of the cornea, resulting in potentially permanent visual deficits. The involvement of ocular glands during corneal infection has been only briefly described. We hypothesized that ocular glands contribute to resistance as an arm of the eye-associated lymphoid tissue and may also be susceptible to infection secondary to microbial keratitis. Utilizing a mouse model of herpes simplex virus 1 (HSV-1) keratitis, we found that infection of corneas resulted in subsequent infection of ocular glands, including harderian glands (HGs) and extraorbital glands. Similarly, infection of corneas with Pseudomonas aeruginosa resulted in secondary infection of ocular glands. A robust immune response, characterized by increased numbers of immune cells and inflammatory mediators, occurred within ocular glands following HSV-1 keratitis. Removal of HGs altered corneal resistance to HSV-1, as measured by increased viral load, decreased corneal edema, and decreased inflammatory cell infiltration. These novel findings suggest that ocular glands are involved in microbial keratitis through their susceptibility to secondary infection and contribution to corneal resistance.IMPORTANCE Microbial keratitis accounts for up to 700,000 clinical visits annually in the United States. The involvement of ocular glands during microbial keratitis is not readily appreciated, and treatment options do not address the consequences of ocular gland dysfunction. The present study shows that ocular glands are susceptible to direct infection by common ocular pathogens, including HSV-1 and Pseudomonas aeruginosa, subsequent to microbial keratitis. Additionally, ocular glands contribute soluble factors that play a role in corneal resistance to HSV-1 and alter viral load, corneal edema, and immune cell infiltration. Further studies are needed to elucidate the mechanisms by which this occurs.

High-throughput screening for phosphatidylserine decarboxylase inhibitors using a distyrylbenzene-bis-aldehyde (DSB-3)-based fluorescence assay

Phosphatidylserine decarboxylases (PSDs) catalyze the decarboxylation of phosphatidylserine to generate phosphatidylethanolamine, a critical step in phospholipid metabolism in both prokaryotes and eukaryotes. Most PSDs are membrane-bound, and classical radioisotope-based assays for determining their activity in vitro are not suitable for high-throughput drug screening. The finding that the PkPSD from Plasmodium knowlesi can be purified in a soluble and active form and the recent development of a fluorescence-based distyrylbenzene-bis-aldehyde (DSB-3) assay to measure PSD activity in vitro have laid the groundwork for screening chemical libraries for PSD inhibitors. Using this assay, here we conducted a high-throughput screen of a structurally diverse 130,858-compound library against PkPSD. Further characterization of the hits identified in this screening yielded five PkPSD inhibitors with IC₅₀ values ranging from 3.1 to 42.3 μm Lead compounds were evaluated against the pathogenic yeast Candida albicans in the absence or presence of exogenous ethanolamine, and YU253467 and YU254403 were identified as inhibiting both native C. albicans PSD mitochondrial activity and C. albicans growth, with an MIC₅₀ of 22.5 and 15 μg/ml without ethanolamine and an MIC₅₀ of 75 and 60 μg/ml with ethanolamine, respectively. Together, these results provide the first proof of principle for the application of DSB-3-based fluorescent readouts in high-throughput screening for PSD inhibitors. The data set the stage for future analyses to identify more selective and potent PSD inhibitors with antimicrobial or antitumor activities.

Light-regulated promoters for tunable, temporal, and affordable control of fungal gene expression

Regulatable promoters are important genetic tools, particularly for assigning function to essential and redundant genes. They can also be used to control the expression of enzymes that influence metabolic flux or protein secretion, thereby optimizing product yield in bioindustry. This review will focus on regulatable systems for use in filamentous fungi, an important group of organisms whose members include key research models, devastating pathogens of plants and animals, and exploitable cell factories. Though we will begin by cataloging those promoters that are controlled by nutritional or chemical means, our primary focus will rest on those who can be controlled by a literal flip-of-the-switch: promoters of light-regulated genes. The vvd promoter of Neurospora will first serve as a paradigm for how light-driven systems can provide tight, robust, tunable, and temporal control of either autologous or heterologous fungal proteins. We will then discuss a theoretical approach to, and practical considerations for, the development of such promoters in other species. To this end, we have compiled genes from six previously published light-regulated transcriptomic studies to guide the search for suitable photoregulatable promoters in your fungus of interest.

Circadian Clearance of a Fungal Pathogen from the Lung Is Not Based on Cell-intrinsic Macrophage Rhythms

Circadian rhythms govern immune cell function, giving rise to time-of-day variation in the recognition and clearance of bacterial or viral pathogens; to date, however, no such regulation of the host-fungal interaction has been described. In this report, we use murine models to explore circadian control of either fungal-macrophage interactions in vitro or pathogen clearance from the lung in vivo. First, we show that expression of the important fungal pattern recognition receptor Dectin-1 ( clec7a), from either bone marrow-derived or peritoneum-derived macrophages, is not under circadian regulation at either the level of transcript or cell surface protein expression. Consistent with this finding, the phagocytic activity of macrophages in culture against spores of the pathogen Aspergillus fumigatus also did not vary over time. To account for the multiple cell types and processes that may be coordinated in a circadian fashion in vivo, we examined the clearance of A. fumigatus from the lungs of immunocompetent mice. Interestingly, animals inoculated at night demonstrated a 2-fold enhancement in clearance compared with animals inoculated in the morning. Taken together, our data suggest that while molecular recognition of fungi by immune cells may not be circadian, other processes in vivo may still allow for time-of-day differences in fungal clearance from the lung.

The second International Symposium on Fungal Stress: ISFUS

The topic of 'fungal stress' is central to many important disciplines, including medical mycology, chronobiology, plant and insect pathology, industrial microbiology, material sciences, and astrobiology. The International Symposium on Fungal Stress (ISFUS) brought together researchers, who study fungal stress in a variety of fields. The second ISFUS was held in May 8-11 2017 in Goiania, Goiás, Brazil and hosted by the Instituto de Patologia Tropical e Saúde Pública at the Universidade Federal de Goiás. It was supported by grants from CAPES and FAPEG. Twenty-seven speakers from 15 countries presented their research related to fungal stress biology. The Symposium was divided into seven topics: 1. Fungal biology in extreme environments; 2. Stress mechanisms and responses in fungi: molecular biology, biochemistry, biophysics, and cellular biology; 3. Fungal photobiology in the context of stress; 4. Role of stress in fungal pathogenesis; 5. Fungal stress and bioremediation; 6. Fungal stress in agriculture and forestry; and 7. Fungal stress in industrial applications. This article provides an overview of the science presented and discussed at ISFUS-2017.

Aspergillus fumigatus Photobiology Illuminates the Marked Heterogeneity between Isolates

The given strain of Aspergillus fumigatus under study varies across laboratories, ranging from a few widely used "standards," e.g., Af293 or CEA10, to locally acquired isolates that may be unique to one investigator. Since experiments concerning physiology or gene function are seldom replicated by others, i.e., in a different A. fumigatus background, the extent to which behavioral heterogeneity exists within the species is poorly understood. As a proxy for assessing such intraspecies variability, we analyzed the light response of 15 A. fumigatus isolates and observed striking quantitative and qualitative heterogeneity among them. The majority of the isolates fell into one of two seemingly mutually exclusive groups: (i) "photopigmenters" that robustly accumulate hyphal melanin in the light and (ii) "photoconidiators" that induce sporulation in the light. These two distinct responses were both governed by the same upstream blue light receptor, LreA, indicating that a specific protein's contribution can vary in a strain-dependent manner. Indeed, while LreA played no apparent role in regulating cell wall homeostasis in strain Af293, it was essential in that regard in strain CEA10. The manifest heterogeneity in the photoresponses led us to compare the virulence levels of selected isolates in a murine model; remarkably, the virulence did vary greatly, although not in a manner that correlated with their overt light response. Taken together, these data highlight the extent to which isolates of A. fumigatus can vary, with respect to both broad physiological characteristics (e.g., virulence and photoresponse) and specific protein functionality (e.g., LreA-dependent phenotypes).

IMPORTANCE

The current picture of Aspergillus fumigatus biology is akin to a collage, patched together from data obtained from disparate "wild-type" strains. In a systematic assessment of 15 A. fumigatus isolates, we show that the species is highly heterogeneous with respect to its light response and virulence. Whereas some isolates accumulate pigments in light as previously reported with strain Af293, most induce sporulation which had not been previously observed. Other photoresponsive behaviors are also nonuniform, and phenotypes of identical gene deletants vary in a background-dependent manner. Moreover, the virulence of several selected isolates is highly variable in a mouse model and apparently does not track with any observed light response. Cumulatively, this work illuminates the fact that data obtained with a single A. fumigatus isolate are not necessarily predictive of the species as whole. Accordingly, researchers should be vigilant when making conclusions about their own work or when interpreting data from the literature.

Seeing the world differently: variability in the photosensory mechanisms of two model fungi

Light plays an important role for most organisms on this planet, serving either as a source of energy or information for the adaptation of biological processes to specific times of day. The fungal kingdom is estimated to contain well over a million species, possibly 10-fold more, and it is estimated that a majority of the fungi respond to light, eliciting changes in several physiological characteristics including pathogenesis, development and secondary metabolism. Two model organisms for photobiological studies have taken centre-stage over the last few decades--Neurospora crassa and Aspergillus nidulans. In this review, we will first discuss our understanding of the light response in N. crassa, about which the most is known, and will then juxtapose N. crassa with A. nidulans, which, as will be described below, provides an excellent template for understanding photosensory cross-talk. Finally, we will end with a commentary on the variability of the light response among other relevant fungi, and how our molecular understanding in the aforementioned model organisms still provides a strong base for dissecting light responses in such species.

Fungal photobiology: visible light as a signal for stress, space and time

Visible light is an important source of energy and information for much of life on this planet. Though fungi are neither photosynthetic nor capable of observing adjacent objects, it is estimated that the majority of fungal species display some form of light response, ranging from developmental decision-making to metabolic reprogramming to pathogenesis. As such, advances in our understanding of fungal photobiology will likely reach the broad fields impacted by these organisms, including agriculture, industry and medicine. In this review, we will first describe the mechanisms by which fungi sense light and then discuss the selective advantages likely imparted by their ability to do so.

Protein kinase A and fungal virulence: a sinister side to a conserved nutrient sensing pathway

Diverse fungal species are the cause of devastating agricultural and human diseases. As successful pathogenesis is dependent upon the ability of the fungus to adapt to the nutritional and chemical environment of the host, the understanding of signaling pathways required for such adaptation will provide insights into the virulence of these pathogens and the potential identification of novel targets for antifungal intervention. The cAMP-PKA signaling pathway is well conserved across eukaryotes. In the nonpathogenic yeast, S. cerevisiae, PKA is activated in response to extracellular nutrients and subsequently regulates metabolism and growth. Importantly, this pathway is also a regulator of pathogenesis, as defects in PKA signaling lead to an attenuation of virulence in diverse plant and human pathogenic fungi. This review will compare and contrast PKA signaling in S. cerevisiae vs. various pathogenic species and provide a framework for the role of this pathway in regulating fungal virulence.

HacA-independent functions of the ER stress sensor IreA synergize with the canonical UPR to influence virulence traits in Aspergillus fumigatus

Endoplasmic reticulum (ER) stress is a condition in which the protein folding capacity of the ER becomes overwhelmed by an increased demand for secretion or by exposure to compounds that disrupt ER homeostasis. In yeast and other fungi, the accumulation of unfolded proteins is detected by the ER-transmembrane sensor IreA/Ire1, which responds by cleaving an intron from the downstream cytoplasmic mRNA HacA/Hac1, allowing for the translation of a transcription factor that coordinates a series of adaptive responses that are collectively known as the unfolded protein response (UPR). Here, we examined the contribution of IreA to growth and virulence in the human fungal pathogen Aspergillus fumigatus. Gene expression profiling revealed that A. fumigatus IreA signals predominantly through the canonical IreA-HacA pathway under conditions of severe ER stress. However, in the absence of ER stress IreA controls dual signaling circuits that are both HacA-dependent and HacA-independent. We found that a ΔireA mutant was avirulent in a mouse model of invasive aspergillosis, which contrasts the partial virulence of a ΔhacA mutant, suggesting that IreA contributes to pathogenesis independently of HacA. In support of this conclusion, we found that the ΔireA mutant had more severe defects in the expression of multiple virulence-related traits relative to ΔhacA, including reduced thermotolerance, decreased nutritional versatility, impaired growth under hypoxia, altered cell wall and membrane composition, and increased susceptibility to azole antifungals. In addition, full or partial virulence could be restored to the ΔireA mutant by complementation with either the induced form of the hacA mRNA, hacAⁱ, or an ireA deletion mutant that was incapable of processing the hacA mRNA, ireA^Δ10. Together, these findings demonstrate that IreA has both HacA-dependent and HacA-independent functions that contribute to the expression of traits that are essential for virulence in A. fumigatus.

Aspergillus fumigatus is the predominant mold pathogen of humans, responsible for life-threatening infections in patients with depressed immunity. The fungus is highly adapted for secretion, a feature that it uses to extract nutrients from the host environment. High rates of protein secretion can overwhelm the protein folding capacity of the endoplasmic reticulum (ER). The resulting ER stress is alleviated by the unfolded protein response (UPR), a signaling pathway that is triggered by the ER-membrane sensor IreA and executed by the downstream transcription factor HacA. This paper uncovers a novel role for IreA in the expression of multiple adaptive traits that allow the fungus to cope with stress conditions that are encountered during infection. Gene expression profiling of ΔireA and ΔhacA mutants revealed that IreA signals predominantly through the canonical IreA-HacA UPR pathway under extreme conditions of ER stress, but has unexpected HacA-dependent and HacA-independent functions even in the absence of ER stress. These findings establish IreA as an important regulator of A. fumigatus pathogenicity and suggest that therapeutic targeting of the dual functions of this protein could be an effective antifungal strategy.

Divergent Protein Kinase A isoforms co-ordinately regulate conidial germination, carbohydrate metabolism and virulence in Aspergillus fumigatus

The genome of Aspergillus fumigatus encodes two isoforms of the catalytic subunit of the cAMP-dependent Protein Kinase (PKA). Although deletion of the class I isoform, pkaC1, leads to an attenuation of virulence, the function of the class II subunit, PkaC2, was previously uninvestigated. In this report, we demonstrate that both isoforms act in concert to support various physiologic processes that promote the virulence of this pathogen. Whereas pkaC1 and pkaC2 single-deletion mutants display wild-type conidial germination, a double-deletion mutant is delayed in germination in response to environmental nutrients. Furthermore, PkaC1 and PkaC2 interact to positively regulate flux through the carbohydrate catabolic pathway and, consequently, the ΔpkaC1ΔpkaC2 mutant is unable to grow on low glucose concentrations. Importantly, the reduced germinative capacity and inability to utilize glucose observed for the ΔpkaC1ΔpkaC2 strain correlated with an inability of the mutant to establish infection in a murine model. Conversely, overexpression of pkaC2 both promotes the in vitro growth on glucose, and restores the fungal burden and mortality associated with the ΔpkaC1 to that of the wild-type organism. Taken together, these data demonstrate the functional capacity of pkaC2 and emphasize the importance of PKA-mediated metabolic control in the pathogenic potential of A. fumigatus.

A role for the unfolded protein response (UPR) in virulence and antifungal susceptibility in Aspergillus fumigatus

Filamentous fungi rely heavily on the secretory pathway, both for the delivery of cell wall components to the hyphal tip and the production and secretion of extracellular hydrolytic enzymes needed to support growth on polymeric substrates. Increased demand on the secretory system exerts stress on the endoplasmic reticulum (ER), which is countered by the activation of a coordinated stress response pathway termed the unfolded protein response (UPR). To determine the contribution of the UPR to the growth and virulence of the filamentous fungal pathogen Aspergillus fumigatus, we disrupted the hacA gene, encoding the major transcriptional regulator of the UPR. The ΔhacA mutant was unable to activate the UPR in response to ER stress and was hypersensitive to agents that disrupt ER homeostasis or the cell wall. Failure to induce the UPR did not affect radial growth on rich medium at 37°C, but cell wall integrity was disrupted at 45°C, resulting in a dramatic loss in viability. The ΔhacA mutant displayed a reduced capacity for protease secretion and was growth-impaired when challenged to assimilate nutrients from complex substrates. In addition, the ΔhacA mutant exhibited increased susceptibility to current antifungal agents that disrupt the membrane or cell wall and had attenuated virulence in multiple mouse models of invasive aspergillosis. These results demonstrate the importance of ER homeostasis to the growth and virulence of A. fumigatus and suggest that targeting the UPR, either alone or in combination with other antifungal drugs, would be an effective antifungal strategy.

The pathogenic mold Aspergillus fumigatus is the leading cause of airborne fungal infections in immunocompromised patients. The fungus normally resides in compost, an environment that challenges the organism to obtain nutrients by degrading complex organic polymers. This is accomplished by secreted enzymes, some of which may also contribute to nutrient acquisition during infection. Extracellular enzymes are folded in the endoplasmic reticulum (ER) prior to secretion. If the folding capacity of the ER is overwhelmed by increased secretory demand, the resulting ER stress triggers an adaptive response termed the unfolded protein response (UPR). In this study, we uncover a previously unknown function for the master transcriptional regulator of the UPR, HacA, in fungal virulence. In the absence of HacA, A. fumigatus was unable to secrete high levels of proteins and had reduced virulence in mice. In addition, loss of HacA caused a cell wall defect and increased susceptibility to two major classes of antifungal drugs used for the treatment of aspergillosis. These findings demonstrate that A. fumigatus relies on HacA for growth in the host environment and suggest that therapeutic targeting of the UPR could have merit against A. fumigatus, as well as other eukaryotic pathogens with highly developed secretory systems.